Fiber-reinforced organic resin matrix composites have a high strength-to-weight ratio or a high stiffness-to-weight ratio and desirable fatigue characteristics that make them increasingly popular as a replacement for metal in aerospace applications where weight, strength, or fatigue is critical. Organic resin composites, be they thermoplastics or thermosets, are expensive today. There is a need for improved manufacturing processes to reduce touch labor and their forming time.
Prepregs combine continuous, woven, or chopped reinforcing fibers with an uncured, matrix resin, and usually comprise fiber sheets with a thin film of the matrix. Sheets of prepreg generally are placed (laid-up) by hand or with fiber placement machines directly upon a tool or die having a forming surface contoured to the desired shape of the completed part or are laid-up in a flat sheet which is then draped and formed over the tool or die to the contour of the tool. Then the resin in the prepreg lay up is consolidated (i.e. pressed to remove any air, gas, or vapor) and cured (i.e., chemically converted to its final form usually through chain-extension) in a vacuum bag process in an autoclave (i.e., a pressure oven) to complete the part.
The tools or dies for metal or composite processing typically are formed to close dimensional tolerances. They are massive, must be heated along with the workpiece, and must be cooled prior to removing the completed part. The delay caused to heat and to cool the mass of the tools adds substantially to the overall time necessary to fabricate each part. These delays are especially significant when the manufacturing run is low rate where the dies need to be changed frequently, often after producing only a few parts of each kind.
In hot press forming, the prepreg is laid-up, bagged (if necessary), and placed between matched metal tools that include forming surfaces that define the internal, external, or both mold lines of the completed part. The tools and composite preform are placed within a press and then the tools, press, and preform are heated. By "preform" we mean the prepreg lay-up.
The tooling in autoclave or hot press fabrication is a significant heat sink that consumes substantial energy. Furthermore, the tooling takes significant time to heat the composite material to its consolidation temperature and, after curing the composite, to cool to a temperature at which it is safe to remove the finished composite part.
As described in U.S. Pat. No. 4,657,717 a flat composite prepreg panel was sandwiched between two metal sheets made from a superplastically formable alloy, and was formed against a die having a surface precisely contoured to the final shape of the part.
Attempts have been made to reduce composite fabrication times by actively cooling the tools after forming the composite part. These attempts have shortened the time necessary to produce a composite part, but the cycle time for and cost of heating and cooling remain significant contributors to overall fabrication costs. Designing and making tools to permit their active cooling increases their cost.
Boeing described a process for organic matrix forming and consolidation using induction heating in U.S. patent application Ser. No. 08/169,655. There, prepregs were laid up in a flat sheet and were sandwiched between aluminum susceptor facesheets. The facesheets were susceptible to heating by induction and formed a retort to enclose the prepreg preform. To ensure an inert atmosphere around the composite during curing and to permit withdrawing volatiles and outgassing from around the composite during the consolidation, we welded the facesheets around their periphery. Such welding unduly impacts the preparation time and the cost for part fabrication. It also ruined the facesheets (i.e., prohibited their reuse which added a significant cost penalty to each part fabricated with this approach). Boeing also described in U.S. patent application Ser. No. 08/341,779 a technique that readily and reliably seals facesheets of the retort without the need for welding and permits reuse of the facesheets in certain circumstances. Our "bag-and-seal" technique applies to both resin composite and metal processing.
An example of a metal forming process combines brazing and superplastic forming of metal with a single induction heating cycle. In U.S. patent application Ser. No. 08/151,433, Boeing describes using a metal pack or retort to contain the multiple sheets in the workpiece with susceptor facesheets that define a pressure zone for the workpiece where we can introduce an inert atmosphere. In the SPF/brazing process, too, we welded the sheets of the retort along their periphery. The welds are costly to prepare, introduce trimming as a necessary step to recover the completed part, and limit the reuse of the retort sheets since they must be shaved smaller when trimming away the weld to recover the completed part. In U.S. patent application Ser. No. 08/341,779, we described resealable bagging systems applicable to this metal forming operation as well.
Similarly in U.S. patent application Ser. No. 08/452,216, Boeing describes the generalized case for combined heat cycle processing as exemplified by the combined SPF/brazing or a combined SPF/.beta.-annealing cycles. Again, the workpiece in these processes is bagged within the external susceptor sheets. The facesheets heat when the induction coil induces eddy currents within them. The facesheets subsequently conduct or radiate their heat to the workpiece.
For purposes of this description, we use "consolidation" to mean both pressure compacting and/or curing of an organic matrix resin through thermally activated chemical reactions to produce a stable composite. By "forming," we mean shaping the composite or metal and retort in its plastic state. "Forming" may entail superplastic forming (SPF), drawing, or some other shaping operation, as those skilled in the art will understand.
The dies or tooling for induction processing are a cast ceramic because a ceramic is not susceptible to induction heating and, preferably, is a thermal insulator (i.e., a relatively poor conductor of heat). The cast ceramic tooling is strengthened and reinforced internally, with inexpensive, strong fiberglass rods or other appropriate reinforcements and externally with metal or other durable strongbacks to permit it to withstand the temperatures and pressures necessary to form, to consolidate, or otherwise to process the composite materials or metals. Ceramic tools cost less to fabricate than metal tools of comparable size and have less thermal mass than metal tooling. Because the ceramic tooling is not susceptible to induction heating, it is possible to embed induction heating elements in the ceramic tooling and to heat the composite or metal retort without significantly heating the tools. Thus, induction heating can reduce the time required and energy consumed to fabricate a part.
While graphite or boron fibers can be heated directly by induction, most organic matrix composites require a susceptor in or adjacent to the composite material preform to achieve the necessary heating for consolidation or forming. A retort of sealed susceptor sheets is also desirable for metal workpieces to control the atmosphere around the workpiece and to achieve uniform heating. The susceptor is heated inductively and transfers its heat principally through conduction to the preform or workpiece that is sealed within the susceptor retort. While the metals in the workpiece may themselves be susceptible to induction heating, the metal workpiece needs to be shielded in an inert atmosphere during high temperature processing to avoid oxidation of the metal, so we usually enclose the workpiece (one or more metal sheets) in a metal retort when using our ceramic tooling induction heating press. Enclosed in the metal retort, the workpiece does not experience the oscillating magnetic field which instead is absorbed in the retort sheets. Heating is by conduction from the retort to the workpiece.
Induction focuses heating on the retort (and workpiece) and eliminates wasteful, inefficient heat sinks. Because the ceramic tools in our induction heating workcell do not heat to as high a temperature as the metal tooling of conventional, prior art presses, problems caused by different coefficients of thermal expansion between the tools and the workpiece are reduced. Furthermore, we are energy efficient because significantly higher percentages of our input energy goes to heating the workpiece than occurs with conventional presses. Our reduced thermal mass and ability to focus the heating energy permits us to change the operating temperature rapidly which improves the products we produce. Finally, our shop environment is not heated as significantly from the radiation of the large thermal mass of a conventional press.
In induction heating for consolidating and/or forming organic matrix composite materials, we place a thermoplastic organic matrix composite preform of PEEK or ULTEM, for example, adjacent a metal susceptor. These thermoplastics have a low concentration of volatile solvents and are easy to use. The susceptor facesheets of the retort are inductively heated to heat the preform. We apply consolidation and forming pressure to consolidate and, if applicable, to form the preform at its curing temperature. Generally, we enclose the preform between two susceptor sheets that are sealed to form a pressure zone. We evacuate the pressure zone in the retort that surrounds the part in a manner analogous to conventional vacuum bag processes for resin consolidation. The retort is placed in an induction heating press on the forming surfaces of dies having the desired shape of the molded composite part. After the retort (and preform) are inductively heated to the desired elevated temperature, we apply differential pressure (while maintaining the vacuum in the pressure zone around the preform) across the retort which, in this case, functions as a diaphragm in the press to form the preform against the die into the desired shape of the completed composite panel.
The retort often includes three metal sheets sealed around their periphery to define two pressure zones. The first pressure zone surrounds the composite panel/preform or metal workpiece and is evacuated and maintained under vacuum. The second pressure zone is pressurized (i.e., flooded with gas) at the appropriate time to help form the composite panel or workpiece. The shared wall of the three layer sandwich that defines the two pressure zones acts as a diaphragm in this situation.
In preparing the retort, as described in U.S. patent application Ser. No. 08/341,779, we often use temporary seals to hold the sheets until the sheets are clamped into the press, and we prefer a "C" spring clamp. The clamp sandwiches the outer susceptor sheets of the retort and provides a compressive force to hold the retort together temporarily, pressing the sheets against an "O" ring gasket. Such a gasket seats between susceptor sheets in a machined groove or crimp around the periphery of adjacent susceptors. For processing below about 600.degree. F. (315.degree. C.), the gasket is generally silicone rubber. Between about 600.degree. F. (315.degree. C.) and 1300.degree. F. (705.degree. C.), the gasket is copper; above about 1300.degree. F. (705.degree. C.), the gasket is stainless steel. The gasket and susceptor sheets abut and form a gas seal via the compressive force of the die set. The "C" clamp permits handling of the retort in and out of the die set. The "C" clamp also provides a current path from the top sheet to the bottom sheet (when the gasket is rubber or stainless steel). The "C" clamp can be omitted when we use a copper gasket, but handling the susceptor sheets is more difficult. The "C" clamp jumper is only required for electrical continuity when the gasket is not an electrical conductor and, then, only on the edges of the retort transverse to the induction coils since the coils induce eddy currents in the susceptor that flow parallel to the coils.
We can perform a wide range of manufacturing operations in our induction heating press. These operations have optimum operating temperatures ranging from about 350.degree. F. (175.degree. C.) to about 1950.degree. F. (1066.degree. C.). For each operation, we usually need to hold the temperature relatively constant for several minutes to several hours while we complete the operations. While we can achieve temperature control by controlling the input power fed to the induction coil, we have discovered a better and simpler way that capitalizes on the Curie temperature of magnetic, electrically conductive materials. By judicious selection of the metal or alloy in the retort's susceptor facesheets and of the driving frequency, we can avoid excessive heating irrespective of the input power. With improved control and improved temperature uniformity in the workpiece, we produce better products.